Being edited. JK who has written a rough draft of a book on this topic, is editing
this material. That which is in italics is by JK. This topic is too important not to be posted as is.

Further reading

We've outlined the structure of a sound deductive
argument, from premises to conclusion. But ultimately, the conclusion of a valid logical argument is only as compelling as
the premises you started from. Logic in itself doesn't solve the problem of verifying the basic assertions which support arguments;
for that, we need some other tool.

The dominant means of verifying basic assertions
is scientific enquiry. However, the philosophy of science and the scientific method are huge topics which are quite beyond
the scope of this document.

For a more comprehensive introduction to logic,
try Flew's "Thinking Straight", listed in the Atheist Media document. A much more detailed book is Copi's "Introduction to Logic". The Electronic Resources document also lists LOGIC-L, a LISTSERV mailing list devoted to discussing the teaching of elementary logic.

Fallacies

There are a number of common pitfalls to avoid
when constructing a deductive argument; they're known as fallacies. In everyday English, we refer to many kinds
of mistaken beliefs as fallacies; but in logic, the term has a more specific meaning: a fallacy is a technical flaw which
makes an argument unsound or invalid.

(Note that you can criticize more than just the
soundness of an argument. Arguments are almost always presented with some specific purpose in mind -- and the intent of the
argument may also be worthy of criticism.)

Arguments which contain fallacies are described
as fallacious. They often appear valid and convincing; sometimes only close inspection reveals the logical flaw.

Below is a list of some common fallacies, and
also some rhetorical devices often used in debate. The list isn't intended to be exhaustive; the hope is that if you learn
to recognize some of the more common fallacies, you'll be able to avoid being fooled by them.

Accent

Accent is a form of fallacy through shifting meaning. In this case, the
meaning is changed by altering which parts of a statement are emphasized. It
is a type of taking a statement out of context.For example:

"He said to Shirley, about our friends, we should not speak ill,
(meaning in their company do not be negative),

and

"He said to Shirley, do not speak ill about our friends.

Be particularly wary of a statement contrary to the beliefs of the speaker.

Ad hoc

As mentioned earlier, there is a difference between argument and explanation. If we're interested in establishing A, and B is offered as evidence, the statement "A because B" is
an argument. If we're trying to establish the truth of B, then "A because B" is not an argument, it's an explanation.

The Ad Hoc fallacy is to give an after-the-fact explanation which doesn't
apply to other situations. Often this ad hoc explanation will be dressed up to look like an argument. For example, if we assume
that God treats all people equally, then the following is an ad hoc explanation:

"I was healed from cancer."

"Praise the Lord, then. He is your healer."

"So, will He heal others who have cancer?"

"Er... The ways of God are mysterious."

Affirmation of the consequent

This fallacy is an argument of the form "A implies B, B is true, therefore
A is true". To understand why it is a fallacy, examine the truth table for implication given earlier. Here's an example:

"If the universe had been created by a supernatural being, we would see order and organization everywhere. And
we do see order, not randomness -- so it's clear that the universe had a creator."

Amphiboly

Amphiboly occurs when the premises used in an argument are ambiguous because
of careless or ungrammatical phrasing. For example:

"Premise: Belief in God fills a much-needed gap."

Anecdotal evidence

One of the simplest fallacies is to rely on anecdotal evidence. For example:

"There's abundant proof that God exists and is still performing miracles today. Just last week I read about
a girl who was dying of cancer. Her whole family went to church and prayed for her, and she was cured."

It's quite valid to use personal experience to illustrate a point; but
such anecdotes don't actually prove anything to anyone. Your friend may say he met Elvis in the supermarket, but those who
haven't had the same experience will require more than your friend's anecdotal evidence to convince them.

Anecdotal evidence can seem very compelling, especially if the audience
wants to believe it. This is part of the explanation for urban legends; stories which are verifiably false have been
known to circulate as anecdotes for years.

Argumentum ad antiquitatem

This is the fallacy of asserting that something is right or good simply
because it's old, or because "that's the way it's always been." The opposite of Argumentum ad Novitatem.

"For thousands of years Christians have believed in Jesus Christ. Christianity must be true, to have persisted
so long even in the face of persecution."

Argumentum ad baculum / Appeal to force

An Appeal to Force happens when someone resorts to force (or the threat
of force) to try and push others to accept a conclusion. This fallacy is often used by politicians, and can be summarized
as "might makes right". The threat doesn't have to come directly from the person arguing. For example:

"... Thus there is ample proof of the truth of the
Bible. All those who refuse to accept that truth will burn in Hell."

"... In any case, I know your phone number and I know
where you live. Have I mentioned I am licensed to carry concealed weapons?"

Argumentum ad crumenam

The fallacy of believing that money is a criterion of correctness; that
those with more money are more likely to be right. The opposite of Argumentum ad Lazarum. Example:

Argumentum ad hominem

Argumentum ad hominem literally means "argument directed at the man"; there
are two varieties.

The first is the abusive form. If you refuse to accept a statement, and
justify your refusal by criticizing the person who made the statement, then you are guilty of abusive argumentum ad hominem.
For example:

"You claim that atheists can be moral -- yet I happen to know that you abandoned your wife and children."

This is a fallacy because the truth of an assertion doesn't depend on the
virtues of the person asserting it. A less blatant argumentum ad hominem is to reject a proposition based on the fact that
it was also asserted by some other easily criticized person. For example:

"Therefore we should close down the church? Hitler and Stalin would have agreed with you."

A second form of argumentum ad hominem is to try and persuade someone to
accept a statement you make, by referring to that person's particular circumstances. For example:

"Therefore it is perfectly acceptable to kill animals for food. I hope you won't argue otherwise, given that
you're quite happy to wear leather shoes."

This is known as circumstantial argumentum ad hominem. The fallacy can
also be used as an excuse to reject a particular conclusion. For example:

This particular form of Argumentum ad Hominem, when you allege that someone
is rationalizing a conclusion for selfish reasons, is also known as "poisoning the well".

It's not always invalid to refer to the circumstances of an individual
who is making a claim. If someone is a known perjurer or liar, that fact will reduce their credibility as a witness. It won't,
however, prove that their testimony is false in this case. It also won't alter the soundness of any logical arguments they
may make.

Argumentum ad ignorantiam

Argumentum ad ignorantiam means "argument from ignorance". The fallacy
occurs when it's argued that something must be true, simply because it hasn't been proved false. Or, equivalently, when it
is argued that something must be false because it hasn't been proved true.

(Note that this isn't the same as assuming something is false
until it has been proved true. In law, for example, you're generally assumed innocent until proven guilty.)

Here are a couple of examples:

"Of course the Bible is true. Nobody can prove otherwise."

"Of course telepathy and other psychic phenomena do
not exist. Nobody has shown any proof that they are real."

In scientific investigation, if it is known that an event would produce
certain evidence of its having occurred, the absence of such evidence can validly be used to infer that the event didn't occur.
It does not prove it with certainty, however.

For example:

"A flood as described in the Bible would require an
enormous volume of water to be present on the earth. The earth doesn't have a tenth as much water, even if we count that which
is frozen into ice at the poles. Therefore no such flood occurred."

It is, of course, possible that some unknown process occurred to remove
the water. Good science would then demand a plausible testable theory to explain how it vanished.

Of course, the history of science is full of logically valid bad predictions.
In 1893, the Royal Academy of Science were convinced by Sir Robert Ball that communication with the planet Mars was a physical
impossibility, because it would require a flag as large as Ireland, which it would be impossible to wave.

Argumentum ad lazarum

The fallacy of assuming that someone poor is sounder or more virtuous than
someone who's wealthier. This fallacy is the opposite of the Argumentum ad Crumenam. For example:

"Monks are more likely to possess insight into the
meaning of life, as they have given up the distractions of wealth."

Argumentum ad logicam

This is the "fallacy fallacy" of arguing that a proposition is false because
it has been presented as the conclusion of a fallacious argument. Remember always that fallacious arguments can arrive at
true conclusions.

"Take the fraction 16/64. Now, cancelling a six on
top and a six on the bottom, we get that 16/64 = 1/4."

"Wait a second! You can't just cancel the six!"

"Oh, so you're telling us 16/64 is not equal to 1/4,
are you?"

Argumentum ad misericordiam

This is the Appeal to Pity, also known as Special Pleading. The fallacy
is committed when someone appeals to pity for the sake of getting a conclusion accepted. For example:

"I did not murder my mother and father with an axe!
Please don't find me guilty; I'm suffering enough through being an orphan."

Argumentum ad nauseam

This is the incorrect belief that an assertion is more likely to be true,
or is more likely to be accepted as true, the more often it is heard. So an Argumentum ad Nauseam is one that employs constant
repetition in asserting something; saying the same thing over and over again until you're sick of hearing it.

On Usenet, your argument is often less likely to be heard if you repeat
it over and over again, as people will tend to put you in their kill files.

Argumentum ad novitatem

This is the opposite of the Argumentum ad Antiquitatem; it's the fallacy of asserting that something is better or more correct simply because it is new, or
newer than something else.

"BeOS is a far better choice of operating system than
OpenStep, as it has a much newer design."

Argumentum ad numerum

This fallacy is closely related to the argumentum ad populum. It consists of asserting that the more people who support or believe a proposition, the more likely
it is that that proposition is correct. For example:

"The vast majority of people in this country believe
that capital punishment has a noticeable deterrent effect. To suggest that it doesn't in the face of so much evidence is ridiculous."

"All I'm saying is that thousands of people believe
in pyramid power, so there must be something to it."

Argumentum ad populum

This is known as Appealing to the Gallery, or Appealing to the People.
You commit this fallacy if you attempt to win acceptance of an assertion by appealing to a large group of people. This form
of fallacy is often characterized by emotive language. For example:

"Pornography must be banned. It is violence against
women."

"For thousands of years people have believed in Jesus
and the Bible. This belief has had a great impact on their lives. What more evidence do you need that Jesus was the Son of
God? Are you trying to tell those people that they are all mistaken fools?"

Argumentum ad verecundiam

The Appeal to Authority uses admiration of a famous person to try and win
support for an assertion. For example:

This line of argument isn't always completely bogus; for example, it may
be relevant to refer to a widely-regarded authority in a particular field, if you're discussing that subject. For example,
we can distinguish quite clearly between:

"Hawking has concluded that black holes give off radiation"

and

"Penrose has concluded that it is impossible to build an intelligent computer"

Hawking is a physicist, and so we can reasonably expect his opinions on
black hole radiation to be informed. Penrose is a mathematician, so it is questionable whether he is well-qualified to speak
on the subject of machine intelligence.

Audiatur et altera pars

Often, people will argue from assumptions which they don't bother to state.
The principle of Audiatur et Altera Pars is that all of the premises of an argument should be stated explicitly. It's not
strictly a fallacy to fail to state all of your assumptions; however, it's often viewed with suspicion.

Bifurcation

Also referred to as the "black and white" fallacy, bifurcation occurs if
someone presents a situation as having only two alternatives, where in fact other alternatives exist or can exist. For example:

"Either man was created, as the Bible tells us, or
he evolved from inanimate chemicals by pure random chance, as scientists tell us. The latter is incredibly unlikely, so..."

Circulus in demonstrando

This fallacy occurs if you assume as a premise the conclusion which you
wish to reach. Often, the proposition is rephrased so that the fallacy appears to be a valid argument. For example:

"Homosexuals must not be allowed to hold government
office. Hence any government official who is revealed to be a homosexual will lose his job. Therefore homosexuals will do
anything to hide their secret, and will be open to blackmail. Therefore homosexuals cannot be allowed to hold government office."

Note that the argument is entirely circular; the premise is the same as
the conclusion. An argument like the above has actually been cited as the reason for the British Secret Services' official
ban on homosexual employees.

Circular arguments are surprisingly common, unfortunately. If you've already
reached a particular conclusion once, it's easy to accidentally make it an assertion when explaining your reasoning to someone
else.

The question presupposes a definite answer to another question which has
not even been asked. This trick is often used by lawyers in cross-examination, when they ask questions like:

"Where did you hide the money you stole?"

Similarly, politicians often ask loaded questions such as:

"How long will this EU interference in our affairs be allowed to continue?"

or

"Does the Chancellor plan two more years of ruinous privatization?"

Another form of this fallacy is to ask for an explanation of something
which is untrue or not yet established.

Fallacies of composition

The Fallacy of Composition is to conclude that a property shared by a number
of individual items, is also shared by a collection of those items; or that a property of the parts of an object, must also
be a property of the whole thing. Examples:

"The bicycle is made entirely of low mass components,
and is therefore very lightweight."

"A car uses less petrochemicals and causes less pollution
than a bus. Therefore cars are less environmentally damaging than buses."

Converse accident / Hasty generalization

This fallacy is the reverse of the Fallacy of Accident. It occurs when you form a general rule by examining only a few specific cases which aren't representative
of all possible cases. For example:

"Jim Bakker was an insincere Christian. Therefore all Christians are insincere."

Converting a conditional

This fallacy is an argument of the form "If A then B, therefore if B then
A".

"If educational standards are lowered, the quality
of argument seen on the Internet worsens. So if we see the level of debate on the net get worse over the next few years, we'll
know that our educational standards are still falling."

Cum hoc ergo propter hoc

This fallacy is similar to post hoc ergo propter hoc. The fallacy is to assert that because two events occur together, they must be causally related. It's
a fallacy because it ignores other factors that may be the cause(s) of the events.

"Literacy rates have steadily declined since the advent
of television. Clearly television viewing impedes learning."

Denial of the antecedent

This fallacy is an argument of the form "A implies B, A is false, therefore
B is false". The truth table for implication makes it clear why this is a fallacy.

Note that this fallacy is different from Non Causa Pro Causa. That has the form "A implies B, A is false, therefore B is false", where A does not in fact
imply B at all. Here, the problem isn't that the implication is invalid; rather it's that the falseness of A doesn't allow
us to deduce anything about B.

"If the God of the Bible appeared to me, personally,
that would certainly prove that Christianity was true. But God has never appeared to me, so the Bible must be a work of fiction."

The fallacy of accident / Sweeping generalization / Dicto simpliciter

A sweeping generalization occurs when a general rule is applied to a particular
situation, but the features of that particular situation mean the rule is inapplicable. It's the error made when you go from
the general to the specific. For example:

"Christians generally dislike atheists. You are a
Christian, so you must dislike atheists."

This fallacy is often committed by people who try to decide moral and legal
questions by mechanically applying general rules.

Fallacy of division

The fallacy of division is the opposite of the Fallacy of Composition. It consists of assuming that a property of some thing must apply to its parts; or that a property of
a collection of items is shared by each item.

"You are studying at a rich college. Therefore you
must be rich."

"Ants can destroy a tree. Therefore this ant can destroy
a tree."

Equivocation / Fallacy of four terms

Equivocation occurs when a key word is used with two or more different
meanings in the same argument. For example:

"What could be more affordable than free software? But to make sure that it remains free, that users can do
what they like with it, we must place a license on it to make sure that will always be freely redistributable."

One way to avoid this fallacy is to choose your terminology carefully before
beginning the argument, and avoid words like "free" which have many meanings.

The extended analogy

The fallacy of the Extended Analogy often occurs when some suggested general
rule is being argued over. The fallacy is to assume that mentioning two different situations, in an argument about a general
rule, constitutes a claim that those situations are analogous to each other.

Here's real example from an online debate about anti-cryptography legislation:

"I believe it is always wrong to oppose the law by
breaking it."

"Such a position is odious: it implies that you would
not have supported Martin Luther King."

"Are you saying that cryptography legislation is as
important as the struggle for Black liberation? How dare you!"

Ignoratio elenchi / Irrelevant conclusion

The fallacy of Irrelevant Conclusion consists of claiming that an argument
supports a particular conclusion when it is actually logically nothing to do with that conclusion.

For example, a Christian may begin by saying that he will argue that the
teachings of Christianity are undoubtedly true. If he then argues at length that Christianity is of great help to many people,
no matter how well he argues he will not have shown that Christian teachings are true.

Sadly, these kinds of irrelevant arguments are often successful, because
they make people to view the supposed conclusion in a more favorable light.

The Natural Law fallacy / Appeal to Nature

The Appeal to Nature is a common fallacy in political arguments. One version
consists of drawing an analogy between a particular conclusion, and some aspect of the natural world -- and then stating that
the conclusion is inevitable, because the natural world is similar:

"The natural world is characterized by competition;
animals struggle against each other for ownership of limited natural resources. Capitalism, the competitive struggle for ownership
of capital, is simply an inevitable part of human nature. It's how the natural world works."

Another form of appeal to nature is to argue that because human beings
are products of the natural world, we must mimic behavior seen in the natural world, and that to do otherwise is 'unnatural':

"Of course homosexuality is unnatural. When's the
last time you saw two animals of the same sex mating?"

Robert Anton Wilson deals with this form of fallacy at length in his book
"Natural Law". A recent example of "Appeal to Nature" taken to extremes is The Unabomber Manifesto.

The "No True Scotsman..." fallacy

Suppose I assert that no Scotsman puts sugar on his porridge. You counter
this by pointing out that your friend Angus likes sugar with his porridge. I then say "Ah, yes, but no true Scotsman
puts sugar on his porridge.

This is an example of an ad hoc change being used to shore up an assertion, combined with an attempt to shift the meaning of the words used original assertion; you might call it a combination of fallacies.

Non causa pro causa

The fallacy of Non Causa Pro Causa occurs when something is identified
as the cause of an event, but it has not actually been shown to be the cause. For example:

"I took an aspirin and prayed to God, and my headache
disappeared. So God cured me of the headache."

Taking a statement as indicative of
a position, when it isnt.

Epicurus maxim states:Pleasure is the highest good, for without
it we do all to get it back.Epicurus was for maximizing the pleasures of the
flesh.

Epicurus goes on to state that the pleasures of the intellect endure the longest and are the purest and thus highest..

Petitio principii / Begging the question

This fallacy occurs when the premises are at least as questionable as the
conclusion reached. Typically the premises of the argument implicitly assume the result which the argument purports to prove,
in a disguised form. For example:

"The Bible is the word of God. The word of God cannot
be doubted, and the Bible states that the Bible is true. Therefore the Bible must be true.

Red herring

This fallacy is committed when someone introduces irrelevant material to
the issue being discussed, so that everyone's attention is diverted away from the points made, towards a different conclusion.

"You may claim that the death penalty is an ineffective
deterrent against crime -- but what about the victims of crime? How do you think surviving family members feel when they see
the man who murdered their son kept in prison at their expense? Is it right that they should pay for their son's murderer
to be fed and housed?"

Reification / Hypostatization

Reification occurs when an abstract concept is treated as a concrete thing.

"I noticed you described him as 'evil'. Where does
this 'evil' exist within the brain? You can't show it to me, so I claim it doesn't exist, and no man is 'evil'."

Shifting the burden of proof

The burden of proof is always on the person asserting something. Shifting
the burden of proof, a special case of Argumentum ad Ignorantiam, is the fallacy of putting the burden of proof on the person who denies or questions the assertion. The
source of the fallacy is the assumption that something is true unless proven otherwise.

"OK, so if you don't think the grey aliens have gained
control of the US government, can you prove it?"

The slippery slope argument

This argument states that should one event occur, so will other harmful
events. There is no proof made that the harmful events are caused by the first event. For example:

"If we legalize marijuana, then more people would start to take crack and heroin, and we'd have to legalize those too. Before long
we'd have a nation full of drug-addicts on welfare. Therefore we cannot legalize marijuana."

Straw man

The straw man fallacy is when you misrepresent someone else's position
so that it can be attacked more easily, knock down that misrepresented position, then conclude that the original position
has been demolished. It's a fallacy because it fails to deal with the actual arguments that have been made.

"To be an atheist, you have to believe with absolute certainty that there is no God. In order to convince yourself
with absolute certainty, you must examine all the Universe and all the places where God could possibly be. Since you obviously
haven't, your position is indefensible."

The above straw man argument appears at about once a week on the net. If
you can't see what's wrong with it, read the "Introduction to Atheism" document.

Tu quoque

This is the famous "you too" fallacy. It occurs if you argue that an action
is acceptable because your opponent has performed it. For instance: